Cathode-ray tube with oxide coated cathode



Patented May 2, 1950 CATHODE-RAY TUBE WITH OXIDE COATED CATHODE Imre Pam, Washington, n.- c.'; H. Russell Bishop administrator No Drawing.-

of said Imre PataLdeceased Application February 26, 1948, Serial No. 11,322 r.

101mm. (01. est-27.5) I

This invention relates to cathode ray tubes having a fluorescent screen and oxide coated cathode; it relates also to the manufacture of such tubes.

In prior methods of manufacturing cathode ray tubes with oxide coated cathode it has been customary to prepare the cathode by coating the metallic core with alkaline earth carbonates, such as by painting or spraying a dispersion of the carbonates onto the metallic core thermally decomposing the carbonates to oxides.

.In this manner it is possible to provide a'thick formation of ion spots occurs more quickly if.

high ve ocity ions bombard the screen. In common television tubes having electron velocities corresponding to 3000-5000 volts the ion spot develops within 50 to 100 hours.

Analyzing the complex phenomenon of the formation of ion spots by noting their position on the fluorescent screen after deflection in a magnetic field of predetermined intensity, I was able to identify the ions and thus determine their source. many of the ions could be eliminated by improved preheating of the electrodes, etc., but that in spite of the most careful preparation of the cathodes and through removal of residual gas the cathodes continued to emit such negative ions. Further investigation of the emission of negative ions from oxide coated cathodes manufactured by different processes revealed: (1) that the residual negative ion emission is a volume effect depending therefore upon the thickness of the oxide layer, and (2) that oxide coated cathodes manufactured by the known vapor method have a lower ion emission than those manufactured by other known methods. The effective life of a cathode is dependent the thickness of its oxide layer and it has bee my observation that cathodes manufactured in accordance with this invention having an oxide layer of microns have an effective life in ex and then These investigations revealed that.

cess of 1000 hours without ion spot formation;

'is more definite, the

and further that cathodes manufactured by the same process but having oxide layers greater than 30 microns and operative under accelerating voltages of about 3000 volts have produced ion spots within 1000 hours operating time. Accordingly, it is an object of this invention to provide a cathode ray tube employing an oxide coated cathode wherein the effective life of the tube without the formation of ion spots on the fluorescent screen is in excess of 1000 hours a electron velocities as high as 5000 volts.

It is a further object of this invention to provide a cathode ray tube employing an oxide coated cathode manufactured by the improved vapor process of this invention wherein the oxide coating is not less than 10 microns and not greater than 30 microns.

By the improved vapor process of this invention the oxide-coated cathodes are made in a separate vessel by the vapor process and then mounted in the cathode ray tubes so that there is no possibility of objectionable condensation of metallic vapors on the various parts of the tubes.

In the vapor process as first proposed by Hertz, a metallic core with an oxide surface, is mounted in a discharge tube; the tube is sealed and evacuated, and then metallic barium or strontium is evaporated in the tube while heating the oxidi ed core. The vaporized barium or strontium reduces the oxidized surface of the metallic core and a very thin surface layer of barium or strontium oxide is produced. While the vapor process produces improved cathodes for use in receiving tubes as compared to other oxide-coated cathodes, in that the emission current is more stable and the saturation current vapor method is subject to a very pronounced disadvantage which has to my knowledge entirely prevented its use in cathode ray tubes. That is, the vaporized alkaline earth metals condense not only on the surface of the cathode but also on all parts of the tube, on the glass surface between the electrodes, and on the fluorescent screen, resulting in troublesome effects; i. e., grid emission, conduction between electrodes, and a source for negative ion emission.

I have found that the vapor process may be used for the manufacture of cathodes for cathode ray tubes by manufacture in a separate vessel by the vapor method and then mounted in the cathode ray tube so that there is no possibility of objectionable condensation of metallic vapor on the parts of the tube. Moreover, by the processit is possible to investigate the quality and tungsten, with a thin 'metal.

thickness of the coating after production in a separate vessel and to use only those which have a layer thickness not in excess of 30 microns. The layer thickness can be regulated by the thickness of the oxidized layer of the metallic core.

According to my improved vapor process the oxidized core metal, which may be tungsten or any of the other core metals known to the art, is placed in a container and a non-oxidizing atmosphere is maintained in the container by evacuation or by introducing an inert gas, such as nitrogen, rare gases, and the like. oxidized core is then heated by direct or indirect heating or high frequency induction heating, and vaporized alkaline earth metals are introduced into the container. The vaporized metals come into contact with the oxidized surface of the core metal, condense on the oxidized Surface to some extent and react therewith and thus reduce the oxidized surface to free metal and form an equivalent amount of the oxides of the alkaline earth metals. After coating of the oxidecoated cathode, it is removed from the container and then mounted in an electron discharge tube. In order to protect the surface of the cathode and to prevent the introduction of objectionable substances, the oxide-coated cathode should be mounted in the cathode ray tube immediately after removal from the container in which it is formed or it should be coated with a protective film prior to or immediately after removal from the container. A good protective film is formed by dipping the oxide-coated cathode into molten paraffin or similar organic compounds immediately after the coated cathode is removed from the container; or preferably by introducing heated inert gas into the container mixed paraffin vapors, for example, which vapors condense upon the cooled cathode to form the protective coating. After mounting the coated cathode in the cathode ray tube, the protective film is readily removed during evacuation of the tube. Any residue remaining after evacuation is not objectionable since it will act as a reducing agent.

Other core metals, in addition to tungsten, which may be used in my method, are platinum, nickel, alloys, etc. It is also within the scope of the invention to coat-the core metal, such as layer of copper or the like, and then to oxidize the copper or equivalent The oxidized copper or the like reacts readily with vaporized alkaline earth metals to form a coating of the oxides of the alkaline earth metals. The surface of the core metal, or the layer of copper or thelike on the core-metal, may be oxidized by any suitable method, such as by heating in an oxidizing atmosphere. By using The copper or the like, the layer thickness can be regulated very accurately.

The core metal is preferably formed in the desired shape of cathode prior to carrying out the steps of my method so that the final layer of a1ka line earth metal oxide is not affected by undue deformation of the cathode. Various types of filamentary cathodes may be made as well as cathodes in the form of a cylinder of thin sheet metal intended for use as indirectly heated cathodes.

/ Any of the alkaline earth metals may be used in my process for producing the final oxide coat ing, but barium or strontium, or mixtures thereof are particularly preferred. These alkaline earth metals may be vaporized by thermal evaporation, cathode sputtering, and the like.

My method is particularly adapted for the coating Of a large number of cathodes in a single In a method of making a cathode-ray tube, the

steps which comprise forming a cathode of a core metal, coating the core metal with a thin layer of copper not to exceed 30 microns, oxidizing the copper, reacting the oxidized copper with. vaporized alkaline earth metal in a non-oxidizing atmosphere to form a coating of the oxide of said alkaline earth metal on said cathode, cooling the resulting oxide-coated cathode, and, then mounting the cathode in a cathode-ray tube.

' IMRE PATAI.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS the spirit and 

